feat: add KMatrixSingleChannel model
K = sum ma Ga(m)/(ma^2 -m^2)
P = sum beta_a ma Ga0 /(ma^2 - m^2)
R(m) = P/(1-i K)
gls coupling include barrier factor.
K = sum ma Ga(m)/(ma^2 -m^2)
P = sum beta_a ma Ga0 /(ma^2 - m^2)
R(m) = P/(1-i K)
gls coupling include barrier factor.